The field of the invention is positron emission tomography (PET) scanners, and particularly volumetric or hybrid PET scanners which organize volumetric PET data as a set of projection planes pxcex8,xcfx86(r,v).
Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. These a re employed as radioactive t racers called xe2x80x9cradiopharmaceuticalsxe2x80x9d by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected in the patient and become involved in such processes as blood flow, fatty acid, glucose metabolism, and protein synthesis. As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two photons, or gamma rays. This annihilation is characterized by two features which are pertinent to PET scannersxe2x80x94each gamma ray has an energy of 511 keV and the two gamma rays are directed in nearly opposite directions. An image is created by determining the number of such annihilations at each location within the field of view.
The PET scanner is cylindrical and includes a detector ring assembly composed of rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by two detectors located on opposite sides of the patient. The number of such simultaneous events (coincidence events) indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. Within a few minutes hundreds of millions of coincidence events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the detector ring. These numbers are employed to reconstruct an image using well-known computed tomography techniques.
When originally developed, PET scanners were strictly multiplanar scanners. In such PET scanners, each detector ring is configured to only detect annihilations occurring within (i.e., within the plane of) that respective ring alone, and not to detect annihilations occurring at other positions within the PET scanner (i.e., annihilations occurring within the other rings of the PET scanner). Because e each detector within each detector ring is capable of receiving photons coming in toward the detector from a variety of angles (rather than merely coming in toward the detector from the center of the ring of which the detector is a part), fixed slice septa are positioned in between each of the detector rings of the PET scanners. The septa, which are commonly composed of lead or tungsten alloy, shield the detectors of each individual detector ring from photons that have not originated from annihilations within the ring, and from scattered photons or other photons that are not resulting from annihilations (i.e., photons entering at either end of the cylindrical PET scanner). Multiplanar data can be organized as a set of two-dimensional sinograms pz(r,xcfx86).
A major innovation in PET scanners that occurred in the late 1980s and early 1990s was the development of volumetric, or true-3D, PET scanners. In contrast to multiplanar scanners, volumetric PET scanners have no septa and consequently the detectors of each detector ring of the scanners can receive photons from a wider range of angles with respect to the plane of the respective ring than in multiplanar PET scanners. Volumetric PET scanners became feasible partly as a result of the increased speed of computers generally, since volumetric PET imaging requires determining the existence of, and processing information related to, coincidence events that occur not merely between pairs of detectors positioned on individual (or adjacent) detector rings, but also between pairs of detectors positioned on detector rings that are spaced more than one ring apart. Volumetric PET scanners allow for increased sensitivity relative to multiplanar scanners, since more coincidence events can be recorded. However, volumetric PET scanners also admit more scattered and random coincidence events to the data set from which the image is reconstructed than multiplanar PET scanners.
Most medium-end and high-end PET scanners available on the market today, including the GE Advance PET scanner manufactured by General Electric Company of Waukesha, Wisconsin, have septa which are automatically retractable. Through the use of such automatically retractable septa, the PET scanners are able to operate as volumetric PET scanners (with the septa retracted) when the benefits associated with increased sensitivity due to volumetric PET scanning outweigh the loss in data quality resulting from the detection of more scattered and random coincidence events. This is typically the case, for example, when the PET scanners are used for brain imaging purposes. However, the PET scanners are also able to operate as multiplanar PET scanners (with the septa extended) when the loss of data quality due to the detection of scattered and random coincidence events becomes excessive. This is typically the case, for example, when the PET scanners are used for body imaging purposes. Thus, PET scanners with automatically retractable septa are xe2x80x9chybridxe2x80x9d PET scanners in that the PET scanners can operate both as multiplanar and volumetric PET scanners depending upon the positioning of the septa.
The amount of data produced by a volumetric PET scanner or a hybrid PET scanner operating in volumetric mode is much greater than that produced by a multiplanar PET scanner. While multiplanar PET data can be described in terms of three variables r,xcfx86 and z and thus be organized as a set of two-dimensional sinograms, volumetric PET data from either a volumetric PET scanner or a hybrid PET scanner typically is described in terms of four variables r, v, xcex8 and xcfx86. Processing of the data from a volumetric PET scanner is therefore much more complicated and time-consuming than processing of the data from a hybrid PET scanner. However, various techniques have been developed to reduce the complexity of, and time required for, processing volumetric PET data.
One such technique is Fourier Rebinning (FORE), which employs an algorithm developed by Michel Defrise and others and is described in publications such as xe2x80x9cA new rebinning algorithm for 3D PET: Principle, implementation and performance.xe2x80x9d by M. Defrise, P. Kinahan and D. Townsend, in Proceedings of the 1995 International Meeting on Fully Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine, pp 235-239, July 1995. The mathematical foundation of the FORE algorithm is an extension of the frequency-distance principle, which relates the two-dimensional discrete Fourier transform (DFT) of the cross-plane sinogram pv,xcex8(r,xcfx86) to the set of two-dimensional sinograms pz(r,xcfx86). That is, through the use of the FORE algorithm, the four-dimensional volumetric PET data can be effectively reduced to a more manageable set of two-dimensional sinograms.
In order to apply the FORE algorithm, it is necessary for the volumetric PET data to be organized as a set of cross-plane sinograms pv,xcex8(r,xcfx86) . In many PET scanners including the GE Advance, however, volumetric PET data is organized as a set of projection planes pxcex8,xcfx86(r,v), with xcex8 as the slowest changing variable. The volumetric PET data is organized in this fashion because the PET scanners are designed to perform corrections on the data in the projection plane format, among other reasons. Because the volumetric PET data is organized in projection plane format, it is necessary to resort the data into the cross-plane sinogram format in order to perform the FORE algorithm on this data. That is, it is necessary to resort data in the form of pxcex8,xcfx86(r,v) into data in the form of pv,xcex8(r,xcfx86).
This resorting of data from the projection plane format into the cross-plane sinogram format is a very time-consuming operation. The resorting of data typically involves writing the data set to disk and then reading it back in sorted form, which is a slow process. Moreover, the volumetric PET data set is typically very large. For example, in the GE Advance PET scanner, a volumetric PET data set includes approximately 25 million elements and requires nearly 100 MB of memory. Because of the size of the volumetric PET data set, the resorting process is particularly time-consuming.
It therefore would be advantageous to develop a method and apparatus for applying the FORE algorithm to data that is originally received in the projection plane format in which it was not necessary to resort the data from the projection plane format into the cross-plane sinogram format.
It has been discovered that an angular portion of the data obtained in the projection plane format can be decimated in a particular manner so that portions of the total amount of data can be read into memory, sorted into sinogram fragments, operated on by way of reduced-length two-dimensional discrete Fourier transforms, and then added together to obtain a set of two dimensional sinograms. The two-dimensional sinograms obtained from the operation are the same set of sinograms as would be obtained by resorting the data from the projection plane format into the cross-plane sinogram format and then performing the FORE algorithm upon that cross-plane sinogram data.
The present invention relates to a method of processing data in a PET scanner in which a projection plane data set is converted into a two-dimensional sinogram data set. The method includes (1) selecting a subset of projection plane data from the projection plane data set, (2) performing a set of Fourier transforms, respectively, on a plurality of data rows within the subset of projection plane data to obtain a transformed data set, and (3) performing an additional set of Fourier transforms on the transformed data set to obtain intermediate results. The additional set of Fourier transforms includes a plurality of individual Fourier transforms respectively corresponding to a plurality of combinations of at least two indices. The method further includes (4) rebinning the intermediate results into a first array, the first array having at least a first dimension corresponding to a physical dimension of the PET scanner, (5) repeating (1)-(4) for each of the remaining subsets of projection plane data, to provide a complete data set within the first array, and (6) performing a set of inverse Fourier transforms with respect to the complete data set to provide the two-dimensional sinogram data set.
The present invention further relates to an additional method of processing data in a PET scanner in which input data in a projection plane format is converted into output data in a two-dimensional sinogram format. The additional method includes (1) performing a Fourier transform of a plurality of data rows within a projection plane data set to obtain a transformed data set, (2) setting a first index, a second index and a third index equal to first, second and third initial values, respectively, and (3) calculating a first array, based upon the values of the first, second and third indices. The method further includes (4) setting a fourth index equal to a fourth initial value, (5) performing a Fourier transform of each column of the first array to obtain a second array, (6) rebinning the data of the second array into a third array, and (7) incrementing the value of the fourth index. The method additionally includes (8) repeating (5)-(7) until the value of the fourth index has reached a fourth limit, (9) incrementing the values of the first, second and third indices, and (10) repeating (3)-(9) until the values of the first, second and third indices have reached first, second and third limits, respectively.
The present invention additionally relates to a PET scanner including a gantry, a plurality of sets of detectors supported by the gantry, and a coincidence detector. The detectors in each set are disposed in a plane and positioned around a central axis that intersects the plane, and the plurality of sets of detectors are spaced along the central axis. The coincidence detector identifies coincidence events from events sensed by the detectors, and is configured to provide coincidence event data in a projection plane format. The PET scanner further includes a processing means for converting the coincidence event data from the projection plane format directly into a two-dimensional sinogram format without conversion of the data from the projection plane format into a cross-plane sinogram format.
The present invention further relates to an array processor including a first array and a second array. In the first array, first and second subsets of transformed row data corresponding respectively to positive and negative values of a first index are stored. The transformed row data is obtained by performing a first set of Fourier transforms with respect to a set of projection plane data. In the second array, the results of a second set of Fourier transforms performed upon the transformed row data are rebinned. The second set of Fourier transforms includes individual Fourier transforms corresponding to each respective combination of a second index and a third index.